The limitation of RF bandwidth resource has stringent restrictions to out-of-band spectrum leakage or adjacent channel power leakage of special RF transmitters working in specified frequency bands. The nonlinear characteristic of RF power amplifier near the saturated working space causes the frequency spectrum of output signals under higher input signal condition higher than the frequency spectrum of the input signals, i.e., the nonlinear characteristic of RF power amplifier results in out-of-band spectrum leakage. Therefore, to depress out-of-band spectrum leakage or adjacent channel power leakage, the RF power amplifier in a communication system usually works in the linear working space, which is far away from the saturated working space. However, that method severely degrades efficiency of RF power amplifier and makes RF power amplifier bulky due to heat dissipation requirement, increasing the cost of RF power amplifier. To solve above problem, an effective measure is to utilize base-band digital pre-distortion technology, which requires establishing a mathematical model for RF power amplifier to calculate pre-distortion first.
To solve sole out-of-band spectrum leakage problem, base-band digital pre-distortion technology is developed for digital mobile communication systems in recent years. The object of that technology is to avoid out-of-band spectrum leakage and low efficiency of RF power amplifiers. The main ideal of base-band digital pre-distortion technology is to carry out preprocessing (i.e., pre-distortion) for input X of RF power amplifier to obtain Xy, which ensures the output Y of RF power amplifier is linear to the original input X; when there is linear relationship between input X and output Y of RF power amplifier, boosting the power output will not cause out-of-band spectrum leakage. Since the pre-distortion is accomplished through base-band digital signal preprocessing, the RF power amplifier shown in FIG. 1 actually represents the entire RF transmission channel from base-band to carrier (up-conversion) and then from carrier to, base-band (down-conversion for signal feedback), including the RF power amplifier.
To solve the ‘pre-distortion’ algorithm in FIG. 1, it is necessary to know the mathematical model of the RF power amplifier; furthermore, that mathematical model should be established specially for solving the pre-distortion algorithm, that is to say, the mathematical model requires minimum calculating workload and memory occupation to solve the pre-distortion algorithm. Such a mathematical model for RF power amplifier is usually a black-box model, i.e., it reflects (Input/Output) I/O relationship of the object (RF power amplifier). As shown in FIG. 2, the difference e between output Y of RF power amplifier and the output Ym of the model is used to refine or correct the model. When e reaches to 0, the model represents the I/O characteristic of RF power amplifier accurately. In actual practice, when e is small enough, it is deemed that the model can reflects I/O relationship of the RF power amplifier at certain accuracy.
Usually, the relationship between input X and output Y of the RF power amplifier comprises amplitude (|X| and |Y|) and phase (Φin and Φout) of base-band digital signal X and Y; wherein the relationship between input amplitude |X| and output amplitude |Y| is referred as AM-AM model of RF power amplifier.|Y|=f(|X|).  (1)
The relationship between input amplitude |X| and I/O phase deviation ΔΦis referred as AM-PM model:Φ=g(|X|),  (2)
herein: Φ=Φout−Φin.
However, for broadband systems, no one has studied AM-PM model successfully yet, even the opinions about the profile of typical AM-PM curve vary; most AM-PM curves in documentation are sketched drawings, which are neither reliable nor supported by experimental data.
Presently, the major part of study work for model of RF power amplifier focuses on establishing AM-AM model for RF power amplifier. Wherein, a typical polynomial model (from PCT patent of PMC (Canada): WO 01/05026 A1) based on I/O amplitude relationship of RF power amplifier (i.e., AM-AM model) involves some delay items of input signal. That model can be used to describe some nonlinear characteristics (e.g., saturation and delay cycle (or, referred as memory)) of RF power amplifier in I/O amplitude aspect. However, that polynomial model has the following disadvantages:
1. The parameter estimation of the polynomial model involves huge calculating workload, which increases exponentially to the order number of the polynomial.
2. After the AM-AM model of RF power amplifier is obtained with the Polynomial method, it is difficult to solve the pre-distortion algorithm corresponding to the polynomial model; especially, when a higher order polynomial model has to be used due to accuracy requirement, it is nearly impossible to obtain the analytic solution of reverse function of the pre-distortion algorithm; as for numerical approach, the calculating workload increases quickly in non-linear manner as the order of the polynomial increases.
3. In a self-adapting pre-distortion system, the model of RF power amplifier has to be stored in the controller; in addition, the output of the model corresponding to input of RF power amplifier has to be calculated online, in order to update the model and the pre-distortion parameters of the self-adapting pre-distortion unit online. However, a polynomial model of RF power amplifier requires a huge storage space, the reason is: in the polynomial approach, as the order of the model increases, the number of the parameters of the model increases; also, a group of new exponential functions will be introduced into the model (the number of the exponential functions depends on the number of delay items in the model). When the model is to be stored, the new parameters have to be stored and a group of tables is required to store those exponential functions (or a multiplier is required), therefore the model storage space is enlarged.
4. The polynomial model (AM-AM model) has been used; however, though many efforts have been exerted in AM-PM model, there is no successful implementation yet. Furthermore, the foundation of arbitrarily correlating input amplitude with I/O phase deviation solitarily in some documents is doubtful.